Prolactin (PRL), growth hormone (GH) and placental lactogen (PL, also called chorionic somatomammotropin) comprise a recognized family of polypeptide hormones that are closely related in structure, function and immunochemistry. All three members of this prolactin-growth hormone family are of similar size (190 to 199 amino acids among various species) and are similar in protein structure. For example, each hormone has a single homologous tryptophan residue at about locus 85 (GH and PL) or 90 (PRL), and two homologous disulfide bonds. The family members also each contain four internal regions of homology which are themselves homologous among the three hormones. Regarding function, all three hormones possess lactogenic and growth-promoting activities. Based upon these observed close structural and functional relationships, it has been postulated that the three hormones arose by duplication of an ancestral hormone gene. See generally, Miller and Eberhardt, 1983, Endocrin. Rev. 4: 97-130.
Due to the medical and veterinary utility of hormones in the prolactin-growth hormone family, the identification of new hormones within this family is desirable.